The present invention relates to the fabrication of semiconductor integrated circuits (IC's). More particularly, the present invention relates to methods and apparatus for reducing charging during plasma processing.
Plasma processing is an important step in the processing of semiconductor wafers, flat panel displays, and other semiconductor components. In a process referred to as "stripping" or "ashing," plasma processing is used to remove photoresist or other material from the surface of a wafer. Plasma processing is also used for etching surface portions of a wafer, and other processes known to those skilled in the art. Although specific plasma processes involving wafers will be referred to herein, the apparatus and method disclosed herein is intended for use in any plasma process where it is desired that charging of the object being processed be reduced. The plasma used in these processes generally includes both ions, which are charged, and reactive free radicals, which are neutral. Ions transport charge to the wafer and in some cases cause a significant charge to build up on the wafer.
An example of a plasma processing chamber 100 is shown in FIG. 1. A plasma source gas 101 is introduced into plasma processing chamber 100 through a gas input port 102 and a gas ring 104. Gas ring 104 is shown as a toroid with a set of gas outlet holes 106 located on the inside of the toroid. Other gas diffusers may also be employed by those skilled in the art, including shower head shaped diffusers or diffusers built into the side of the chamber itself. A plasma generating element 108 creates an electromagnetic field which causes plasma source gas 101 to form a plasma. Plasma generating element 108 is shown as a Transformer Coupled Plasma (TCP) coil, but plasma generating element 108 may also be any other type of plasma generating element including inductively coupled Radio Frequency (RF), Helicon, Electron Cyclotron Resonance (ECR), or microwave. Plasma generating element 108 is connected to RF power source 110, or any other appropriate power source for the specific plasma generating element 108 being used, which provides the power for generating the plasma.
The plasma is generated in a plasma generating region 112, which is in proximity to gas ring 104 and plasma generating element 108. The approximate boundary of plasma generating region 112 within plasma processing chamber 100 is shown by a dashed line 113. The plasma diffuses from plasma generating region 112 into a plasma diffusion region 114, where the plasma reacts with a wafer 116. Wafer 116 is held by a wafer transport arm 118, or alternatively may be held on a chuck. The chuck can be electrically grounded or connected to a power source. To keep pressure in plasma processing chamber 100 low, the plasma and byproduct material is exhausted through an exhaust port 120.
Although a wafer has been described as being processed in plasma processing chamber 100, plasma processing chamber 100 may also be used to process flat panel displays or other semiconductor components which require etching, stripping, ashing, or other forms of plasma processing. Throughout this specification and the claims, when the term "wafer" is used, it should be understood to include wafers or any component which may undergo plasma processing.
As noted above, the plasma is made up of both charged ions and reactive free radicals. The reactive free radicals react with materials on the surface of the wafer but do not carry charge to the wafer. The ions, however, carry charge to the wafer, and, when the wafer does not have a good electrical connection to ground or a potential source, a substantial charge may build up either on the entire wafer or on certain devices on the wafer.
In certain plasma processes, such as anisotropic etching, the charged property of the ions is useful. A voltage source is connected to the wafer which accelerates the ions towards the wafer and causes anisotropic, or directional etching to occur. In other processes, the free radicals and not the ions are more desirable for performing the important plasma processing function. This is the case in some resist stripping processes. In some cases, therefore, the ions may not be essential to the plasma process and may in fact be detrimental because they can destroy structures on the wafer by charging the wafer.
Charging of the wafer is a well known problem which can lead to degradation of device performance. This is especially a problem when there are large antennas of conducting material such as metal or polysilicon on a wafer attached to a single transistor gate which utilizes a very thin (&lt;100 .ANG.or &lt;50 .ANG.) gate oxide. The large antennas collect relatively large amounts of charge by virtue of their large area and funnel the charge down to the small transistor. Performance degradation or destruction often results. A process where such an antenna is exposed to a plasma including ions for a long period of time such as in the overetching portion of a resist removal step causes particular problems with devices.
Attempts to remove ions from the plasma have been made. Insulating quartz baffles have been used within plasma processing chamber 100 to remove ions with some success, but insulating baffles in the prior art allow a significant portion of the ions to pass.
A method and apparatus are needed for alleviating the wafer charging problem.